Ocean and sea fish – which are very nutritious and important part of the human diet – are more expensive than ever. Unfortunately, they will become rarer and more costly in the near future. 

A new study at Tel Aviv University (TAU) points to a strong “edge effect” in marine protected areas (MPAs) in many places around the world, sharply reducing by 60% the fish population living up to one to 1.5 km within the MPA, compared to the core areas of the waters. This significantly diminishes the effectiveness of the MPA.

The study was conducted by Sarah Ohayon, a doctoral student at the laboratory of Prof. Yoni Belmaker at the School of Zoology in TAU’s  Wise Faculty of Life Sciences and TAU’s Steinhardt Museum of Natural History. The study was recently published in the Nature Ecology & Evolution journal under the title “A meta-analysis reveals edge effects within marine protected areas.” 

MPAs were designed to protect marine ecosystems and help conserve and restore fish populations and marine invertebrates whose numbers are increasingly dwindling due to overfishing. The effectiveness of MPAs has been proven in thousands of studies conducted worldwide. At the same time, most studies sample only the “inside” and “outside” of the MPAs. 

There remains too little scientific information about what happens in the space between the core of the MPAs and areas open for fishing around them. 

Ohayon explained that when an MPA functions properly, the expectation is that the recovery of the marine populations in the MPAs will result in a spillover, a process where fish and marine invertebrates migrate outside the borders of the MPA. In this way, the MPA can contribute not only to the conservation of marine nature, but also to the renewal of fish populations outside the MPA that have dwindled due to overfishing. 

To answer the question what is the dominant spatial pattern of marine populations from within MPAs to areas open for fishing around them, the researchers conducted a meta-analysis that included spatial data of marine populations from dozens of MPAs located in different parts of the oceans.  

“When I saw the results, I immediately understood that we are looking at a pattern of edge effect,” recalled Ohayon. “The edge effect is a well-studied phenomenon in terrestrial protected areas, but surprisingly has not yet been studied empirically in MPAs. This phenomenon occurs when there are human disturbances and pressures around the MPA, such as hunting/fishing, noise or light pollution that reduce the size of natural populations within the MPAs near their borders.” 

The researchers found that 40% of the “no-take” MPAs (areas where fishing activity is completed prohibited) around the world are less than one square kilometer, which means that entire area is likely to experience an edge effect. In total, 64% of all no-take MPAs in the world are smaller than 10 square kilometers and may hold only about half (45-56%) of the expected population size in their area compared to a situation without an edge effect. These findings indicate that the global effectiveness of existing no-take MPAs is far less than previously thought.

It should be emphasized that the edge effect pattern does not eliminate the possibility of fish spillover, and it is quite plausible that fishers still enjoy large fish coming from within the MPAs. This is evidenced by the concentration of fishing activity at the borders of MPAs. At the same time, the edge effect makes it clear to us that marine populations near the border of MPAs are declining at a faster rate than the recovery of the populations around the MPA.

The study findings also show that in those MPAs with buffer zones around them, no edge effect patterns were recorded, but rather a pattern consistent with fish spillover outside the MPA. Additionally, a smaller edge effect was observed in well-enforced MPAs than those where illegal fishing was reported. 

“These findings are encouraging, as they signify that by putting buffer zones in place, managing fishing activity around MPAs and improving enforcement, we can increase the effectiveness of the existing MPAs and most probably also increase the benefits they can provide through fish spillover,” added Ohayon.

 “When planning new MPAs, apart from the implementation of regulated buffer zones, we recommend that the no-take MPAs targeted for protection be at least 10 km2 and as round as possible. These measures will reduce the edge effect in MPAs. Our research findings provide practical guidelines for improving the planning and management of MPAs, so that we can do a better job of protecting our oceans.”